In the field of construction, polymeric sheets, which are often referred to as membranes or panels, are used to cover flat and low-sloped roofs. Commonly used materials for membranes include thermoplastics such as plasticized polyvinylchloride (p-PVC), thermoplastic olefins (TPE-O, TPO), and elastomers such as ethylene-propylene diene monomer (EPDM). Such membranes are typically delivered to a construction site in rolls, transferred to the place of installation and unrolled and positioned. The roof substrate on which the membrane is adhered may be comprised of variety of materials depending on the installation site. The roof substrate may be e.g. a concrete, metal, or wood deck, or it may include an insulation board or recover board and/or an existing membrane.
The membranes are secured to the roof substrate to provide sufficient mechanical strength to resist the shearing forces applied on it e.g. due to high wind loads. In addition to securing the membrane to the substrate, the edges of adjacent membranes are overlapped and the overlapping portions are heat welded or bonded to each other to create a waterproof seal. The technique for bonding the overlapped portions of the membranes depends on the materials of the membrane and roof substrate, typically the seams are sealed using adhesives, adhesive tapes or by heat-welding. The membrane can be mechanically fastened to the roof substrate using screws and barbed plates.
Mechanical fastening enable high strength bonding but they provide direct attachment to the roof substrate only at locations where a mechanical fastener affixes the membrane to the surface, which makes mechanically attached membranes susceptible to flutter.
Membranes can also be secured to the roof substrate by adhesive attachment, which allows the formation of fully adhered roofing system. In this case the majority, if not all, of the membrane is secured to the roof substrate. Fully adhered roofing systems can be prepared by using a number of techniques. Typically a contact bonding method is employed, in which both the membrane and the surface of the roof substrate are coated with a solvent or water based contact adhesive after which the membrane is contacted with the surface of the substrate. The volatile components of the contact adhesive are “flashed off” to provide a partially set adhesive prior to contacting the membrane with the substrate.
According to another technique, fully adhered roofing system is prepared by using membranes having a pre-applied adhesive layer on the bottom of the membrane. In this case, the membrane is provided to the construction site containing a pre-applied layer of adhesive covered with a release liner. At the time of use the release liner is removed and the membrane is secured to the substrate without using additional adhesives. Membranes having a pre-applied adhesive layer covered by release liner are known as peel and stick membranes. In fully adhered roofing systems the seams between overlapping parts of adjacent membranes can be adjoined by using the same or different adhesive as for membrane bonding, by using sealing tapes or by heat welding.
Fully adhered roofing systems provide certain advantages over mechanically fastened systems including easy installation and high bond strength between the membrane and the roof substrate. Adhesive based bonds are, however, sensitive to thermal and moisture degradation. Normally the adhesive bond is in contact with only minor amount of moisture since the membrane protects the below roofing structures against water penetration. However, even a minor breach in the membrane will allow significant amount of water into the system. Such damages in the membrane may be generated by extensive traffic across the roof surface storing of heavy equipment on the roof e.g. during façade cleaning.
Once the damage is discovered, the repair of the membrane typically consists of patching the opening and thereby leaving the moisture trapped in the system. In a typical system, the trapped moisture will degrade the adhesive bond and/or the cohesive strength of the top surface of the insulation or cover board below causing localized delamination of the assembly and making the roof susceptible to significant damage under wind load. One of the disadvantages of the state-of-the-art adhered roofing systems is that the layer of adhesive is exposed to moisture degradation in case of leaking membrane causing localized delamination under high wind loads.